时隔多年终于写出来一个递归，小开心0v0
题目：
https://www.codewars.com/kata/541c8630095125aba6000c00
In this kata, you must create a digital root function.
A digital root is the recursive sum of all the digits in a number. Given n, take the sum of the digits of n. If that value has more than one digit, continue reducing in this way until a single-digit number is produced. This is only applicable to the natural numbers.
Here's how it works:
digital_root(16)
=> 1 + 6
=> 7
digital_root(942)
=> 9 + 4 + 2
=> 15
=> 1 + 5
=> 6
digital_root(132189)
=> 1 + 3 + 2 + 1 + 8 + 9
=> 24 ...
=> 2 + 4
=> 6
digital_root(493193)
=> 4 + 9 + 3 + 1 + 9 + 3
=> 29 ...
=> 2 + 9
=> 11 ...
=> 1 + 1
=> 2
解：
public static int digital_root(int n) {
// ...
int sum = 0;
int ten = 10;
int i = 0;
do {
int b = n%ten;
n = n/10;
if(i>5)
i = 0;
sum += b;
//System.out.println(sum);
i++;
}while(n>0);
if(sum>=10) {
sum = digital_root(sum);
}
//System.out.println(sum);
return sum;
}